Is your sensor the TDS0045 ?Where can I find more information ?Perhaps here, http://www.dynament.com/infrared-sensor-data/infrared-gas-sensor-applications.htm

Why did you power it with 3.3V, I think they can be used with 5V.Are you sure that it uses RS232 levels ? Or is it using serial 5V TTL levels ?

Are you sure the sensor itself contains all the logic for serial communication ?I think the document for communication protocol is for the OEM board.The sensor itself seems to be analog.I hope you didn't damage the sensor or the OEM board with high voltage levels from the MAX3232.

The schematic is to make RS232 high voltage levels, to be able to connect the sensor to the RS232 serial port of a PC.

You don't have to make your own MAX232 or MAX3232 circuit. If you search Ebay for "rs232 ttl" you will find many adapters to make TTL (5V) level serial signals. It's always good to have something like that, in case you want to connect the sensor to a PC.

Explanation: If you take a look of the datasheet of the MAX232, the pins 8,13,7,14 are the high level signals, and the 9,10,11,12 pins are the 5V TTL level pins. Now take a look at the schematic they gave you, and you see that the 5V TTL level signals are connected to the sensor.

To connect it to the Arduino, forget the MAX232. Use the 3.3V (perhaps 5V is possible), and connect sensor RX to a Arduino SoftwareSerial TX pin, and sensor TX to SoftwareSerial RX pin.